Analog-to-digital (A/D) converters are employed to convert an analog signal received as an input to a digitally coded signal provided as an output. The analog input may be provided as a voltage or a current. The A/D converter may operate on a current, voltage or charge basis. The various types of A/D converters result in trade-offs between speed of completing the conversion versus the amount of hardware needed to achieve the conversion. At one end of this spectrum is the successive approximation A/D converter. Successive approximation A/D's operate to sequentially determine bits of the digital code corresponding to the analog input. Successive approximation A/D converters generally require one clock cycle per bit of resolution.
At the other end of the A/D spectrum is a parallel converter, generally referred to as a "flash" converter, that is capable of great speed due to the presence of sufficient hardware to complete the entire analog-to-digital conversion process simultaneously rather than sequentially. Flash converters generally require only one clock cycle to complete the entire analog-to-digital conversion process. Between these two ends of the A/D converter spectrum are a variety of hybrid and algorithmic converters. Flash A/D's usually employ a voltage applied across a precision resistor string. Intermediate taps at the resistor junctions, as well as in some cases at the resistor-potential junctions, are coupled to one input each of the plurality of comparators whose other inputs are tied in common. When the sampled analog input signal is applied to the commoned comparator input, the output states of the plurality of comparators are decoded to provide a digitally coded output word representative of the sampled analog input signal.
In a conventional flash A/D converter, the number of resistors forming the resistor string, as well as the number of comparators, required to provide an n-bit converter is substantially 2.sup.n -1. It can be seen that as the number of bits increase, the number of comparators and resistors increases more rapidly. Various techniques exist to reduce the number of resistors in the resistor string, and also the number of comparators, from 2.sup.n -1 to some smaller number, depending upon the reduction technique employed and the division between the number of most significant bits and the number of least significant bits.
Regardless of the topology of a flash A/D converter, however, the power expended in the conversion process will be the sum of the switching power and the quiescent power of each individual comparator. In some designs, the comparator quiescent power can be significant. Therefore, while flash A/D converters are considered preferred in terms of their speed, the power consumption may be unacceptable for many situations.
A need remaining in the prior art, therefore, is for a flash A/D converter that consumes less power, while not sacrificing the rapid conversion rate associated with flash converters.